1. 1
   A quality inspector at a chemical supplier tests a sample of white powder, believed to be pure sodium chloride, by heating it slowly in a water bath and recording the temperature at which it starts to melt and the temperature at which melting is complete.

   (a)

   In chemistry, a pure substance is best described as:

   [1 mark]

   • AA single element or compound, not mixed with any other substance
   • BAny substance that looks clean and has no visible dirt in it
   • CA mixture of two or more elements or compounds combined in fixed proportions
   • DAny liquid that has been filtered to remove solid particles

   (b)

   The inspector finds that the sample melts sharply at exactly the expected melting point of pure sodium chloride, over a very narrow temperature range. If the sample had instead been impure, what would the inspector most likely have observed?

   [1 mark]

   • AMelting at a temperature higher than the true melting point of pure sodium chloride, over a very narrow range
   • BMelting at exactly the same sharp temperature as the pure substance
   • CNo melting at all, regardless of impurities present
   • DMelting over a wider range of temperatures, starting below the true melting point of pure sodium chloride

   (c)

   Explain how melting point and boiling point data can be used to determine whether a substance is pure or impure.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A pharmaceutical company manufactures a cough medicine that contains several different ingredients, each added for a specific purpose: the active drug to treat symptoms, a flavouring to make it palatable, a preservative to extend shelf life, and a thickening agent to give the correct consistency.

   (a)

   This type of mixture, designed as a useful product, is called:

   [1 mark]

   • AA pure compound
   • BA formulation
   • CAn isotope
   • DAn alloy only

   (b)

   Besides medicines, which of the following is also a good example of a formulation, designed as a mixture with each component serving a specific purpose?

   [1 mark]

   • AA single element such as pure copper metal
   • BPure distilled water, containing only H2O molecules
   • CPaint, containing pigment, solvent and binder mixed for a specific purpose
   • DPure oxygen gas collected in a gas jar

   (c)

   Explain why the cough medicine is described as a formulation rather than a pure substance, giving two examples of formulations other than medicines.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An art restorer investigates the ink used to write a historical document by carrying out paper chromatography. She places a small spot of the ink near the bottom of a strip of chromatography paper, dips the bottom edge of the paper into a solvent, and allows the solvent to travel up the paper over several minutes.

   (a)

   An art restorer investigates the ink used in a historical document by carrying out paper chromatography. She places a small spot of ink near the bottom of a strip of chromatography paper, dips the bottom edge in a solvent, and allows the solvent to travel up the paper. Explain, in terms of a stationary phase and a mobile phase, how paper chromatography separates the different substances in the ink.

   [3 marks]

   (b)

   On the restorer's chromatogram, one component of the ink travels 6.0 cm from the baseline while the solvent front travels 8.0 cm from the baseline in the same time. Calculate the Rf value for this component, showing your working, and explain how the restorer could use Rf values, along with the number of spots produced, to help determine whether the ink is a pure single dye or a mixture of dyes.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A food safety laboratory is investigating a batch of imported cooking oil that customers have complained tastes unusual, suspecting it may have been deliberately diluted with a cheaper, different oil to increase profits. The laboratory manager is also training new scientists to test food products, many of which are formulations.

   (a)

   Explain how the laboratory could use melting point or boiling point data, together with paper chromatography, to investigate whether the cooking oil is pure or has been mixed with another substance, and discuss the strengths and limitations of these methods.

   [6 marks]

   (b)

   The laboratory manager explains to trainee scientists that formulations, such as many food products, are deliberately designed mixtures, and this makes assessing their purity more complicated than for a simple pure compound. Explain the difference between a pure substance and a formulation, and discuss why testing the purity of a formulation, such as a food product, is more complex than testing a single pure substance for purity.

   [6 marks]

   Total for question 4: 12 marks

End of questions